1. Field of the Invention
The present invention is related to an improved method for the reduction of nitrous oxide; and more particularly, to ammonia-mediated reduction of nitrous oxide.
2. Description of the Prior Art
Nitrous oxide (N2O) is not commonly considered an atmospheric pollutant and has not been considered a constituent of the gaseous pollutants collectively referred to as nitrogen oxides (NOx) which have received wide attention as pollutants harmful to the environment. However, recent studies indicate that N2O in the Earth's atmosphere may be increasing at a rate of about 0.2% per year and that this increase appears to be caused by anthropogenic activity.
N2O is a major stratospheric source of NO, is believed to be involved in destroying the ozone layer and is recognized to be a greenhouse gas. Because N2O has an atmospheric lifetime of approximately 150 years, researchers are attempting to identify sources of the pollutant and to limit further production of the harmful gas. Recent reports such as an article by Thiemens and Trogler, Science, 251 (1991) 932 suggest that various industrial processes significantly contribute to the increased levels of N2O found in the Earth's atmosphere.
For example, nitrous oxide is a by-product formed during the manufacture of monomers used in producing 6,6- and 6,12-nylon. Nylon polymers are typically formed by subjecting a dicarboxylic acid and a diamine to a condensation polymerization reaction. The most widely used dicarboxylic acid, adipic acid, is prepared primarily by oxidizing cyclohexane in air to form a cyclohexanol/cyclohexanone mixture followed by oxidizing such mixture with HNO3 to form adipic acid and N2O. Thiemens and Trogler calculate that about 1 mol of N2O per mole of adipic acid is formed as a side product in adipic acid processes. Assuming that 2.2×109 kg of adipic acid are produced globally per year, about 1.5×1010 mol yr−1 of N2O by-product or 10% of the annual output of atmospheric N2O can be attributed to this single process. Also, for many industrial processes, N2O may be co-present with nitrogen oxides, NOx (NO and NO2), in the effluent gases.
M. Schiavello and coworkers, (J. Chem. Soc. Faraday Trans. 1, 71(8), 1642-8) studied various magnesium oxide-iron oxides and magnesium oxide-iron oxide-lithium oxide systems as N2O decomposition catalysts. While magnesium oxide-iron oxide samples which were fired in air and which contained Mge2O4 demonstrated low activity, similar samples fired under reducing atmospheres and containing Fe2+ in solid solution demonstrated greater activity. The researchers concluded that Fe3+ ions in the ferrite phase are not catalytically active toward the subject reaction whereas Fe3+ ions contained in MgO together with Li+ are catalytically active when the ratio of lithium to iron is less than 1.
P. Porta and coworkers (J. Chem. Soc. Faraday Trans 1, 74(7), 1595-603) studied the structure and catalytic activity of CoxMg1-xAl2O4 spinel solid solutions for use as catalysts in decomposing N2O into gaseous nitrogen and oxygen. The catalytic activity per cobalt ion in various N2O decomposition catalysts was found to increase with increasing dilution in MgO. The distribution of cobalt ions among octahedral and tetrahedral sites in the spinel structure of CoxMg1-xAl2O4 was found to vary with temperature and the fraction of cobalt ions in octahedral sites was found to increase with increasing quenching temperature. The researchers concluded that catalytic activity generally increases as a greater amount of cobalt ions is incorporated into octahedral sites in the structure.
W. Reichle (Journal of Catalysis 94 (1985) 547) reported that various anionic clay minerals belonging to the pyroaurite-sjogrenite group, such as hydrotalcite (Mg6Al2(OH)16(CO32−).4H2O can be thermally decomposed to form a product which is a useful catalyst for vapor-phase aldol condensations. Replacement of Mg by Fe, Co, Ni and Zn and/or replacement of Al by Fe and Cr also results in isomorphous double hydroxides which, on heat treatment, are rendered catalytically active. The reference also states that the activity of the catalyst is strongly affected by the temperature at which the hydrotalcite is activated.
Commonly owned U.S. Pat. No. 5,171,553 discloses a highly efficient, commercially viable process for removing N2O from gaseous mixtures. The process utilizes catalysts comprising a crystalline zeolite which, at least in part, comprise five membered rings having a structure type selected from the group consisting of BETA, MOR, MUI, MEL and FER wherein the crystalline zeolite has been at least partially ion-exchanged with a metal selected from the group consisting of copper, cobalt, rhodium, iridium, ruthenium and palladium.
Likewise, commonly owned U.S. Pat. No. 5,407,652 discloses an efficient catalytic pollution control process for removing N2O from gaseous mixtures. The process utilizes catalysts derived from anionic clay minerals such as hydrocalcites, sjogrenites and pyroaurites which, after appropriate heat activation, provide superior N2O decomposition activity.
While the prior art has shown an awareness of the decomposition of N2O into its respective components, industry urgently needs to develop enhanced catalytic processes for destroying N2O emissions prior to the venting of commercial process effluent streams into the atmosphere. This need is particularly critical with respect to effluent streams containing low levels of this contaminant. In addition, methods are needed to remove this contaminant from engine exhaust streams. It would be particularly useful if the catalytic decomposition of N2O could be combined with reduction of NOx so as to economically and efficiently remove these pollutants from both industrial effluent streams and engine exhaust streams.